CG-5 Easy polar alignment using the polar scope

By Pawel Lancucki, Pawel.Lancucki@pl.ibm.com

Accurate polar alignment is very useful for high-power visual observations and is essential for time astrophotography. The easiest and least accurate method of polar alignment is sighting along the mount RA axis and making it approximately pointing towards Polaris. The most accurate but also time consuming is the drift method, where one assess the mount displacement from being polar aligned by observing drift of stars observed in the high-power eyepiece. Somewhere in between, there are polar alignment scopes. They offer reasonable accuracy (actually, you may need to refine polar alignment using the drift method, but it is quicker if you start being fine aligned instead of rough) and are very quick to use.

However, there are two provisions:

• the polar alignment scope need to be exactly parallel to the mount mechanical RA axis
• you need to properly set the position of the scope reticle based on your time zone and you must include various other corrections.

I have received a polar alignment scope for my CG-5 mount in a separate box and without any instructions, so I had to solve both problems.

First, I had to collimate the optical axis of my polar alignment scope with the mount RA axis. The procedure to do this is quite straightforward:

1. Set the mount on the tripod without the scope or counterweights attached. Remove polar scope covers and rotate DEC axis until you see opening in it.

2. Plug the polar alignment scope into the lower–end opening and secure it with thumbscrews.

3. Peer through the polar scope eyepiece – you will see a reticle with a circle, crosshairs and small circlet where one of the crosshairs touches the large circle.

4. Using altitude and azimuth controls, set the cross to be centered on a distant object. You can use a terrestrial landmark, but you will need to perform a lot of adjustment in altitude, so I prefer to center the crosshairs on Polaris – it won’t move far enough providing that the collimation is completed within few minutes.

5. Turn the RA axis 180 degrees and lock it. Ideally, Polaris should still stay at the crosshairs. If it moved, the polar alignment scope need collimation.

6. Using three thumbscrews, move the cross-hairs half-way from their initial position to the current position of Polaris.

7. Repeating steps 5 and 6 will lead to an accurate collimation of the polar alignment scope and the mount RA axis in just 3 – 4 iterations – unless you collimate in a wrong way.

8. Once the polar alignment scope is collimated, you can use it to speed-up setting of your mount for photography!

Below I described a quite convenient and quick method of polar aligning your mount without the calculations and corrections.

1. Print the picture below and get it laminated. This will be your polar alignment quick reference!

2. Set the mount on the tripod without the scope or counterweights attached. Remove polar scope covers and rotate DEC axis until you see opening in it.

3. Sighting along the mount RA axis, set it approximately polar aligned – if you were careful you should see Polaris in the polar alignment scope field of view.

4. Keeping the picture in your hands, turn it to match as closely as possible the relative locations of circumpolar constellations on the sky – the two most important are Cassiopeia and Ursa Major.

5. The red arrow points the direction of displacement of Polaris from the true North Pole. However, the polar alignment scope reverses the view so actually Polaris should get into the red circle!

6. Turn the RA axis of the mount until the small circlet matches location of the red circle on the picture.

7. Using altitude and azimuth controls, set Polaris to be centered in the small circlet.

8. Voila – providing that reticle was turned in a right position, you got a reasonably good initial polar alignment – could be better than 5 arc min.

9. Now you can use drift method to refine your polar alignment.

Just a final remark:

Rough polar alignment can be achieved without the polar scope mounted – you can simply peer through the opening and adjust the altitude and azimuth controls until Polaris is reasonably centered in the field of view.

Note. The picture included was created using TheSky software from Software Bisque.